In digital spread spectrum (DSS) communication, a wide band carrier signal is modulated by a narrow band message signal. The wide-band carrier is typically generated by modulating a single frequency carrier using a pseudo-random noise (P/N) code sequence. The data rate at which a message is communicated is usually much lower than the P/N code symbol or “chip” rate. The ability of DSS to suppress interference is proportional to a ratio of the chip rate to data rate. In many applications, there are thousands of code chips per data bit. There are two basic types of DSS systems: direct sequence spread spectrum systems (DSSS) and frequency hop spread spectrum systems (FHSS).
The DSSS systems spread the signal over a bandwidth fRF±Rc, where fRF represents the center bandpass carrier frequency and Rc represents the PN-code maximum chip rate, which in turn is an integer multiple of the symbol rate Rs. Multiple access systems employ DSSS techniques when transmitting multiple channels over the same frequency bandwidth to multiple receivers, each receiver having its own designated PN-code. Although each receiver receives the entire frequency bandwidth only the signal with the receiver's matching PN-code will appear intelligible, the rest appears as noise that is easily filtered. The DHSS system PN-code sequence spreads the data signal over the available bandwidth such that the carrier appears to be noise-like and random, but is deterministic to a receiver using the same PN-code. These systems are well known in the art and will not be discussed further.
FHSS systems employ a PN-code sequence generated at the modulator that is used in conjunction with an m-ary frequency shift keying (FSK) modulation to shift the carrier frequency fRF at a hopping rate Rh. A FHSS system divides the available bandwidth into N channels and hops between these channels according to the PN-code sequence. At each frequency hop time a PN generator feeds a frequency synthesizer a sequence of n chips that dictates one of 2 n frequency positions. The receiver follows the same frequency hop pattern. FHSS systems are also well known in the art and need not be discussed further.
At the receiver, a carrier replica is generated by reducing the DSS signal to baseband and multiplying it with a locally generated replica of the original narrow-band carrier using a local oscillator. If the frequency and phase of the carrier replica is the same as that of the received original narrow-band carrier, then the multiplier output signal will be the product of the bipolar P/N code and intended message. The P/N code is removed by multiplying the wide-band data stream with the locally generated replica of the P/N code that is time aligned with the received P/N code. The de-spreading process of generating the carrier replica with proper carrier frequency and phase and generating the P/N code replica at the proper rate and time offset is a complex problem. In many DSS communication systems, the necessary carrier frequency, carrier phase, and P/N code offset are not known a priori at the receiver, which tries different values until a large signal is observed at the data-filter output. This is termed the search or acquisition process, and a DSS signal is said to be acquired when the proper frequency, phase, and code offset have been determined.
The above cross-referenced patent applications also detail various concerns for security in DSS communication systems, along with exemplary wireless environments in which a secure DSS system is advantageously deployed. One such security concern is low probability of intercept (LPI) and low probability of detection (LPD) by adverse parties of messages sent over the secure DSS system. Once a secure spreading code is known to an unauthorized user, some or all of the messages over the system may be compromised, and the breach may not be known immediately to the authorized parties. Further, in some communication systems such as space-based satellite DSS systems that may or may not be a secure system, certain hardware systems must be hardened against ambient radiation such as alpha particles. Typically, the hardware of concern is made to resist some minimal level of radiation and the overall system employs triple redundancy to ensure against failure of any single or pair of like components. This is an expensive proposition, both in the radiation hardening, in the redundancy of components, and in the additional weight to be launched into space. By reloading often, some of this redundancy may be eliminated.
Frequency hopping is known in the art. Adaptive modulation is also known in the art of multiple-input/multiple output communication systems, but this is generally not considered a hopping technique as modulation is changed in response to channel conditions rather than according to a predetermined schedule or sequence. It is also known in the art to use code hopping as an encryption technique for securing communications in a DSS system. For example, U.S. Pat. No. 6,657,985 to Su-Won Park, entitled “Orthogonal Code Hopping Multiple Access Communication System”, describes a system that divides channels according to hopping patterns of the orthogonal codes allotted to the respective channels. In an illustrated embodiment, a first orthogonal code OC hops three times for every bit stream duration, a second OC hops once per bit stream duration, and a third OC hops at multiples of the bit stream duration (n=2 in FIG. 4). The hopping code on each channel then repeats over the fraction or multiple of the bit stream duration. While advantageous in reducing probability of detection, this is seen to expose the secure communication system to compromise if an adverse party should gain access to a transmitter or receiver. This results in an increase to probability of interception, because each transmitter and receiver has a hopping controller or a memory that has the hopping pattern stored within. Interrupting the hopping repetition is not seen to resolve this security risk, as all hopping patterns and codes are seen to be stored and subject to breach if the hardware is compromised.
What is needed in the art is a cost-effective way to enhance security in a DSS communication system without inordinately spending bandwidth.